1. Field of the Invention
The present invention relates to a multi-beam scanning apparatus adapted for use in an image forming apparatus such as a printer, a facsimile machine, a copier, etc., and more particularly, to a multi-beam scanning apparatus having a lateral magnification which is capable of enhancing effectiveness of a light source and minimizing a fluctuation in pitch of light on a scanned surface.
2. Description of the Related Art
Generally, a light scanning apparatus of an image forming apparatus such as a printer, a facsimile machine or a copier uses a light source having a plurality of light emitting parts that generate a multi-beam, such as laser beams, in order to form an electrostatic latent image at high speed on a photosensitive body, such as a photosensitive drum or a photosensitive belt.
Such a light scanning apparatus forms the electrostatic latent image on the photosensitive body by the process of: converting the laser beams from the plurality of light emitting parts, such as laser diodes, of the light source into parallel rays of light having predetermined intervals through a collimator lens; leading the laser beams to a light deflector having deflecting reflection surfaces that rotate at high speed; deflecting the direction of the laser beams at the deflecting reflection surfaces; and emitting the laser beams onto the photosensitive body through a scanning lens such as an f-θ lens to form a plurality of scan lines.
Referring to FIG. 1, there is schematically illustrated a conventional multi-beam scanning apparatus forming an electrostatic latent image on a photosensitive body.
The light scanning apparatus includes a light source side optical system 10 having a light source 1 including a plurality of light emitting parts 1a and 1b (FIG. 2) such as laser diodes to emit laser beams, collimator lens 2 arranged to correspond to the light emitting parts 1a and 1b, a slit 8 through which the laser beams which have passed through the collimator lens 2 are converted into a predetermined form, and a cylindrical lens 3 through which the laser beams which have passed through the slit 8 are imaged into elongated linear lights with respect to a main scanning direction A and focused with respect to a sub-scanning direction B. The conventional light scanning apparatus also includes a light deflector 4 having deflecting reflection surfaces 4a supported on a motor (not shown) to be rotated at high speed, to deflect the direction of the laser beams emitted from the cylindrical lens 3; and a scanning optical system 20 including a lens system 5 having first and second f-θ lenses 5a and 5b that compensate for the error included in the laser beams deflected from the light deflector 4, an elongated curvature-of-image-field correcting lens 6 correcting a curvature of image field of the laser beams passed through the first and second f-θ lenses 5a and 5b, and a reflective mirror 9 reflecting the laser beams passed through the curvature-of-image-field correcting lens 6, onto a scanned surface on a photosensitive body such as a photosensitive drum 7.
The operation of the conventional multi-beam scanning apparatus constructed as above will be described below.
The laser beams, which are modulated in accordance with the input image signals, are emitted from the light emitting parts 1a and 1b of the light source 1, and converted into parallel, or collected rays of light by the collimator lens 2.
Then, after passing through the slit 8 that shapes the laser beams in a predetermined form, the laser beams are passed through the cylindrical lens 3, and then deflected by the deflecting reflection surfaces 4a of the light deflector 4, which is rotated at high speed by the motor.
Next, the laser beams are passed through the first and second f-θ lenses 5a and 5b and the curvature-of-image field correcting lens 6, are reflected by the reflective mirror 9, and are then condensed as light spots to scan a plurality of scan lines onto the scanned surface of the photosensitive drum 7 along the main scanning direction.
At this time, the photosensitive drum 7 is driven to rotate in the sub-scanning direction by a driving motor (not shown). Accordingly, as a result of the scanning movements of the light spots in the main scanning direction and the rotation of the photosensitive drum 7 in the sub-scanning direction, a predetermined electrostatic latent image is formed on the photosensitive drum 7.
However, the conventional multi-beam scanning apparatus operated as above generally requires that a composite lateral magnification β in the sub-scanning direction B of the optical systems 10 and 20, i.e., from the light source 1 to the scanned surface, satisfies the condition of 2≦β≦8.5.
More specifically, the composite lateral magnification β in the sub-scanning direction B is determined by a lateral magnification in the sub-scanning direction B of the light source side optical system 10 including the collimator lens 2 and the cylindrical lens 3 and a lateral magnification in the sub-scanning direction B of the scanning optical system 20 including the first and second f-θ lenses 5a and 5b and the curvature-of-image-field correcting lens 6.
However, if the lateral magnification in the sub-scanning direction B of the scanning optical system 20, i.e., from the light deflector 4 to the scanned surface, is greater than 2, a magnification aberration on the scanned surface is increased, and thereby a performance fluctuation of the light spots is increased. Therefore, usually, the composite lateral magnification β in the sub-scanning direction B mainly depends on an imaging magnification of the collimator lens 2 and an imaging magnification of the cylindrical lens 3.
Accordingly, if the composite lateral magnification β in the sub-scanning direction B is less than 2, a focal length of the collimator lens 2 and a focal length of the cylindrical lens 3 become too small, so that the scanning optical system 20 must be very close to the light deflector 4. As a result, the scanning optical system 20, for example, the curvature-of-image-field correcting lens 6, is disposed close to the scanned surface, thereby becoming easily contaminated by the dispersion of developer.
Also, since the composite lateral magnification β in the sub-scanning direction B corresponds to the ratio of a gap d, i.e., pitch, between the light emitting parts 1a and 1b to a pitch of scan lines, if the composite lateral magnification β in the sub-scanning direction B is greater than 8.5 at an appropriate pitch of the scan lines, the pitch of the light emitting parts 1a and 1b becomes small, thereby generating a thermal cross-torque phenomenon such that the adjacent light emitting parts 1a and 1b have a thermal effect on each other.
Accordingly, the conventional light scanning apparatus is designed so that the composite lateral magnification β in the sub-scanning direction B satisfies the condition of 2≦β≦8.5 to allow the curvature-of-image-field correcting lens 6 to be disposed apart from the scanned surface, thereby preventing the curvature-of-image-field correcting lens 6 from being easily contaminated by dispersion of developer, and to prevent the thermal cross-torque phenomenon. The multi-beam scanning apparatus satisfying such a condition is disclosed in Japanese patent laid-open No. 1998-54950.
On the other hand, as semiconductor manufacturing techniques are rapidly developed, even though the composite lateral magnification β in the sub-scanning direction B is designed to satisfy the condition of 8.5≦β, the thermal cross-torque phenomenon resulting when the pitch of the light emitting parts 1a and 1b is too small does not occur.
However, if a multi-beam scanning apparatus is designed to satisfy the condition of 8.5≦β, a problem may occur in the lateral magnification in the sub-scanning direction B of the light source side optical system 10, i.e., the focal lengths of the collimator lens 2 and the cylindrical lens 3, particularly the focal length of the collimator lens 2, is lengthened.
Thus, when the focal length of the collimator lens 2 is lengthened, the amount of the laser beams, which are actually imaged on the scanned surface through the slit 8 among the laser beams emitted through the collimator lens 2 from the light source 1, is reduced and thus becomes very small. As a result, an output of the light source 1 must be increased, or a light source having a large output must be used, thereby increasing manufacturing costs.